The present invention relates to a low cost method for producing methanol from coal or other source of carbon utilizing an ocean thermal energy conversion (OTEC) plantship. More specifically, the present invention relates to a low cost method for producing methanol and other hydrocarbon fuels utilizing carbon-free sources of hydrogen and pure oxygen available from water electrolysis on an OTEC plantship. The unique advantages of the present invention are that the low cost hydrogen and oxygen which can be produced by use of inexhaustible solar energy absorbed in the surface waters of the tropical oceans allow methanol and derivative hydrocarbon fuels to be produced at minimum cost and the method of producing the methanol does not produce carbon dioxide as a by-product.
The world is entering into an era of increasing shortages of natural sources of energy. Conservative estimates indicate that if in one hundred years the world per capita energy demand is only two-thirds of current U.S. per capita consumption and if population-control techniques are successful, the world will require 3000 quads (3.times.10.sup.18 BTU) of energy per year as compared with current world consumption of approximately 300 quads per year. This is a ten-fold increase in one hundred years.
It is apparent that (1) the demand for transportation fuels will not be met by crude oil and natural gas resources, and (2) it will be necessary to develop a worldwide synfuel industry utilizing coal as the primary source of carbon. However, two major problems are associated with the establishment of a coal-based synthetic fuel industry to meet the world future demands. The first problem is that CO.sub.2 levels in the atmosphere are rising at a rate that is linearly proportional to the combustion of carbon. A substantial body of scientific opinion predicts that, if the concentration of atmospheric CO.sub.2 continues to increase at this rate, deleterious climatic changes could occur as early as the year 2030. The second and more immediate problem is the deficiency of hydrogen in coal. In simplified terms, the hydrogen-carbon ratio of good synthetic fuel is approximately 2.0 whereas the hydrogen-carbon ratio in the average coal deposit is approximately 0.8. Because of the lack of hydrogen the inefficient synfuel process throws away excess carbon in the form of CO.sub.2 and coke. As can be appreciated, the thrown away CO.sub.2 and the thrown away coke is a waste of valuable carbon. In the more efficient Fischer-Tropsch process, hydrogen is obtained from the water molecule by an exchange process (2C+2H.sub.2 O.fwdarw.CH.sub.4 +CO.sub.2). This also aggravates the problem of atmospheric CO.sub.2 and of course is also a waste of valuable carbon.
As can be appreciated, the solution to the problem of adding excess carbon to the atmosphere in the form of CO.sub.2 is to have a major source of carbon free hydrogen.
One of the most attractive synfuels is methanol because it can be used directly as a motor fuel in internal combustion engines or it may be converted into hydrocarbon fuel. For example, the Mobil process involving dehydration of methanol can be used to produce hydrocarbon fuel at over 95% efficiency (Mobil Methanol to Gasoline Process, David Liederman et al, 15th IECEC Conf., Seattle, Wash., Aug. 18-22, 1980). Methanol is also a preferred fuel for fuel cells of the molten carbonate type, in which over 60% conversion of the heating value (HHV) of the fuel to electric power has been demonstrated.
Methanol is conventionally made at the present time from natural gas by a process which involves the following steps:
______________________________________ -.DELTA.H kcal/gmol ______________________________________ (1) CH.sub.4 + H.sub.2 O .fwdarw. CO + 3H.sub.2 -49.3 (2) CO + 2H.sub.2 .fwdarw. CH.sub.3 OH 21.6 (3) CH.sub.4 + 2O.sub.2 .fwdarw. CO.sub.2 + 2H.sub.2 O 191.8 ______________________________________
The first reaction, as shown, is endothermic and reaction (3) is, therefore, necessary to supply heat for the process. In the normal commercial process, this means that, as an example, 32,000 standard cubic feet (SCF) of natural gas may be consumed per ton of methanol produced. Because of this and the increasing cost of natural gas, considerable effort is being directed towards finding economic alternatives for preparing methanol.
The use of OTEC plantships for the production of methanol solves two major problems. First, the electrolysis of water to produce carbon-free hydrogen decreases the amount of CO.sub.2 injected into the atmosphere and second, the electrolysis of water can be accomplished economically by utilizing an inexhaustible source of energy. The OTEC plantships comprise energy producing systems which exploit the difference in temperature between the surface and deep ocean waters to run a Rankine engine or the equivalent and thereby generate electric power. Regions having an average temperature differential (.DELTA.T) of 20.degree.-25.degree. C. are particularly attractive for OTEC plants. The highest .DELTA.T's and the smallest seasonal variations in .DELTA.T are noted in the tropical areas of the Pacific and Atlantic Oceans and these areas are consequently particularly attractive for OTEC plants.
OTEC plantships are ships which may be moored but usually cruise slowly in the selected tropical ocean area to generate electric power by means of a turbine generator driven via a closed Rankine cycle. Since the temperature difference which is relied upon is dependent on solar energy absorbed at the ocean surface, it is evident that OTEC plants offer an essentially inexhaustible energy source for the production of electricity. It is estimated, for example, that with 325 MW.sub.e plantships spaced at 30-mile intervals, the total electric power produced on board would be around 10 million megawatts, 40 times present U.S. electric power generation.
A further advantage of OTEC plantships is that the electric power generated thereon may be used to make various chemical products, e.g., ammonia, which may be stored aboard the ship and delivered at appropriate ports when and as desired. Typically, OTEC electric power may be used to electrolytically produce H.sub.2 and O.sub.2 from ocean water, the gases thus obtained being available for appropriate reactions.
Substantial effort has also been directed towards obtaining synthetic fuels ("synfuels") by pyrolysis of coal and char oxidation. One such system is that known as the char oil energy development (COED) process developed by FMC Corporation (see "Char Oil Energy Development", Vol 1, J. F. Jones et al, FMC Corp. for U.S. Dept. of Commerce, Rept. FE-1212-T-9; and "The Pyrolysis Route to Gasification", R. Tracy Eddinger, Reprint #8412-0513-2/79/0779, A. Chem. Soc.).
In the conventional COED system, bituminous coal is subjected to pyrolysis to produce oil, gas and char. The char may then be converted to liquid fuel by (1) introduction of steam and O.sub.2 into a vessel containing the char where reaction occurs to form CO, H.sub.2 ; followed by (2) reaction of the CO with H.sub.2 to form methanol, the latter being subsequently dehydrated in a further step, e.g., by the Mobil process, to form gasoline (CH.sub.2).sub.n and water.
The conventional COED process, which is entirely land-based, requires the use of pure O.sub.2, which must be supplied by an air liquefaction plant thus adding significantly to the overall cost of oxidation step (1). Additionally, overall efficiency is reduced because roughly half of the char has to be oxidized to CO.sub.2 to provide heat for the reaction: EQU C+H.sub.2 O.fwdarw.CO+H.sub.2
which is endothermic and to provide additional H.sub.2 as needed via the reaction: EQU C+2H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2
which is also endothermic.
It is therefore one object of this invention to provide a method of producing methanol and derivative hydrocarbon fuels by use of inexhaustible electrical energy from the ocean for production of pure hydrogen and oxygen at minimum cost.
It is another object of this invention to provide a method of producing methanol and derivative hydrocarbon fuels utilizing a low cost source of hydrogen which does not require carbon as a feed stock.
It is a further object of this invention to provide a method of producing methanol and derivative hydrocarbon fuels without injecting waste CO.sub.2 into the atmosphere.
It is still another object of this invention to provide a method of producing methanol and derivative hydrocarbon fuels by combining efficient land based procedures and efficient OTEC plantship procedures.